ADS824

10

SBAS076A

2Vp-p

NOTE: R

V

IN

R

2

200

Ω

R

1

1k

Ω

ADS824

R

S

50

Ω

10pF

0.1µF

IN

IN

CM (+2.5V)

INT/EXT

R

F

499

Ω

R

IN

499

Ω

V

REFB

(+1.5V)

REFT

(+3.5V)

1/2

OPA2681

1/2

OPA2681

R

F

1k

Ω

50

Ω

0.1µF

0.1µF

RSEL

+V

S

+5V

+5V

FIGURE 4. DC-Coupled Interface Circuit with Level Shifting Using Dual Current-Feedback Amplifier OPA2681.

FIGURE 5. Transformer Coupled Input.

SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION

(Transformer Coupled)

If the application requires a signal conversion from a single-

ended source to feed the ADS824 differentially, a RF trans-

former might be a good solution. The selected transformer

must have a center tap in order to apply the common-mode

DC voltage necessary to bias the converter inputs.

AC grounding the center tap will generate the differential

signal swing across the secondary winding. Consider a step-

up transformer to take advantage of a signal amplification

without the introduction of another noise source. Further-

more, the reduced signal swing from the source may lead to

an improved distortion performance.

The differential input configuration may provide a notice-

able advantage of achieving good SFDR performance over

a wide range of input frequencies. In this mode, both inputs

of the ADS824 see matched impedances, and the differential

signal swing can be reduced to half of the swing required for

single-ended drive. Figure 5 shows the schematic for the

suggested transformer-coupled interface circuit. The com-

ponent values of the R-C low-pass may be optimized de-

pending on the desired roll-off frequency. The resistor across

for good power transfer and Voltage Standing Wave Ratio

(VSWR).

REFERENCE OPERATION

Figure 6 depicts the simplified model of the internal refer-

ence circuit. The internal blocks are the bandgap voltage

reference, the drivers for the top and bottom reference, and

V

IN

IN

IN

CM

22

Ω

22

Ω

47pF

R

T

47pF

+

10µF

0.1µF

INT/EXT

RSEL

+5V

ADS824

1:n

0.1µF

R

G

FIGURE 6. Equivalent Reference Circuit with Recommended

Reference Bypassing.

ADS824

REFT

ByT

CM

ByB

REFB

Bypass Capacitors: 0.1µF || 2.2µF each

Bandgap Reference and Logic

V

REF

400

Ω

400

Ω

400

Ω

400

Ω

+1

+1

+V

S

50k

Ω

50k

Ω

INT/EXT

RSEL